It’s missing because Windows doesn’t occupy itself too much with integrating all of the OpenGL functions. This is really the driver manufacturer’s job.
On macOS, Apple conveniently supplies all of the GL headers and libs easily because, well, they like their little walled garden. It’s a similar situation on Linux, except there’s a form of folder abstraction to access the right headers and libs to link to.
Unfortunately, JUCE’s OpenGLExtensionFunctions is lacking, and basically collecting dust. I’ve had a similar set of issues as you, where many (!) functions are missing. And so I made this FR: FR: Wrangle a comprehensive set of OpenGL functions & macros